Centrifugal sludge separator with desludging control means

ABSTRACT

In a self-cleaning liquid-solids centrifugal separator having a drum wherein a piston is displaced periodically to expose a sludge discharge orifice through which sludge is discharged without stopping rotation of the drum, a second piston is provided. The second piston defines a chamber with the first piston which is filled with a control fluid. The second piston is displaced as the weight of sludge increases and when it moves a predetermined extent the flow of control fluid causes said first piston to move and expose said discharge opening. Displacement of the second piston can also set in operation a control mechanism which, either during each cycle or after a predetermined number of cycles, drains control fluid from the system and permits complete discharging of sludge from the drum even after the weight of sludge on the second piston is below the level at which it would otherwise cause the first piston to return to the position wherein the discharge orifice is closed.

[451 July 31,1973

United States Patent 1 Hemfort 887,232 8/1943 France.............................. 233/20 A 54] CENTRIFUGAL SLUDGE SEPARATOR wrrn nsswncmo CONTROL MEANS Primary Examiner-George I-l. Krizmanich Attarney-Burgess, Dinklage 8L Sprung [75] Inventor: Heinrich Hemiort, Oelde, Germany [7 3] Assignee: Westtalia Separator AG, Westfalia,

Germany ABSTRACT liquid-solids centrifugal separator having a drum wherein a piston is displaced periodically to expose a sludge discharge orifice through which sludge is discharged without stopping rotation of in a self-cleaning [22] Filed: Apr. 30, 1971 [21] Appl. No.: 139,078

the drum, a second piston is provided. The second piston defines a chamber with the first piston which is filled with a control fluid. The second piston is dis- [30] Foreign Application Priority Data May 6, 1970 Germany................... P 20 22 l98.l

placed as the weight of sludge increases and when it moves a predetermined extent the flow of control fluid causes said first piston to move and expose said discharge opening. Displacement of the second piston can also set in operation a control mechanism which, either during each cycle or after a predetermined number of cycles, drains control fluid from the system and permits complete discharging of sludge from the drum even [56] References Clter'll/ ENTS Lyskovtsov UNITED STATES PA 3,371,859 3/1968 233/20 A after the weight of sludge on the second piston is below 3,494,546 1970 /2 A the level at which it would otherwise cause the first pis- 3,4l5,446 12/1968 wellandmu 233/20 R ton to return to the position wherein the discharge orifice is closed.

FOREIGN PATENTS OR APPLICATIONS 707,651 3/1966 italy 233/20 A 8 Claims, 5 Drawing Figures PATENTEDJUL31 I975 sum 1 or 5 FIG. I.

INVENTOR HEINRICH HEMFORT BY UJRGESS. DINKLAGE. & SPRUNG ATTORNEYS.

PATENIEDJUU 3, 749 303 sum 2 OF 5 I FIG. 2.

INVENTOR HEINRICH HEMFORT BY BURGESS. DINKLAGE 8. SPRUNG ATTORNEYS PATENTED 1 3,749,303

sum 3 or 5 INVENTOR HEINRICH HEMFORT BY BURGESS. DINKLAGE & SPRUNG ATTORNEYS PAIENHEDJUL31 I913 3, 749.303

SHEET t 0F 5 FIG; 4;

INVENTOR HEINRICH HEMFORT BY BURGESS. DINKLAGE 8 SPRUNG ATTORNEYS.

PATENTEDJUL3I I975 SHEET 5 OF 5 FIG.

INVENTOR HEINRICH HEMFORT BY BURGESS. DINKLAGE & SPRUNG ATTORNEYS.

CENTRIFUGAL SLUDGE SEPARATOR WITH DESLUDGING CONTROL MEANS The invention relates to a self-cleaning drum for a centrifugal sludge separator having a hydraulically operated control piston which, upon the filling of a chamber with hydraulic fluid, closes an orifice through which solids in the drum can be discharged so as to retain solids separated from the fluid being processed; when the control fluid is drained from the chamber, the piston is displaced to open the discharge orifice for discharge of the solids. These actions are carried out at full drum speed.

Control systems are known in which the centrifuging time is preset and in which all the necessary operations for desludging are carried out on a pre-set time schedule.

Automatic controllers are also known which, when the sludge chamber is filled with solids, transmit a signal to a control apparatus to start the desludging process. Controllers of this kind either monitor the clear phase or sense the sludge level in the drum. These nontimed controllers have the advantage of eliminating the need to estimate the length of the centrifuging cycles on the basis of the solid content of the fluid being separated, the throughput of the separator and the sludge holding capacity of the drum; consequently changes in the solids content of the feed and in the throughput rate during the course of operation no longer pose problems.

Automatic controllers are known, for example, from German Pats. Nos. 718,739, 1,145,100, 1,173,030, 1,186,414 and 1,275,959.

Apparatuses monitoring the clear phase have the disadvantage that they do not respond until the clarifying action of the separator has already diminished considerably.

To sense the sludge level, a sensing fluid is generally fed into the sludge chamber. Apparatuses of this kind have the disadvantage that, in the case of granular and fibrous solids which have been separated from singlephase liquids, they suffer from a certain unreliability, because solids of this kind do not ensure a complete elogging of the passages which handle the sensing fluid, so that no impulse for initiating desludging is created.

Centrifugal separator drums of the self-cleaning type can be operated with partial desludging or full desludging or a combination of both.

In partial desludging, the piston that closes the discharge apertures forthe solids is only partially opened or is opened only for a brief period of time. In this manner only a portion of the solids collecting in the sludge chamber is discharged and a sludge residue is retained in the drum to prevent the liquid from escaping the inner space of the drum. In this kind of operation it is not necessary to stop the feed of the fluid being separated.

In complete desludging the drum is completely emptied. To prevent losses of the fluid being separated, the input of the latter is shut off before the desludging operation. It is also known in the art to displace the liquid in the drum with another liquid of greater specific weight prior to desludging.

It is an object of the present invention to provide a self-cleaning centrifugal separator which will effect partial desludging automatically.

Another object of the invention is to provide a selfcleaning centrifugal separator which will effect complete desludging automatically in combination with a control system outside of the separator.

These and other objects are realized in accordance with the present invention wherein to effect partial desludging, above the first piston which opens and closes the sludge discharge orifices there is placed a second or auxiliary piston. The pressure of the drum load acts on the upper face of the second piston and the lower face is acted upon by a pressurized control fluid in a chamber between the two pistons; the axial movement of the second piston is controlled by the relative forces on its faces and automatically effects alternate filling and emptying or partial emptying of the control fluid in the control chamber located beneath the first piston causing the first piston to move in its closing position and opening position respectively.

To limit the force of the drum load on the second piston the upper part of the latter can be covered partially by a cap which is sealed from the interior of the drum by means of packing. Instead of or in addition to said cap, helical springs may be uniformly distributed over the circumference between the first and second piston to supplement the pressure of the control fluid. Axial displacement of the second piston can also be used to generate an impulse for a control circuit to permit complete desludging either during each cycle or, preferably, after a pre-determined number of cycles.

The invention will be further described with reference to the accompanying drawings, wherein:

FIG. 1 is an axial section through a separator in accordance with the invention, the separator including a drum having a valve for releasing the control fluid from a control chamber which causes the first piston to move in its open position by shutting off the input of control fluid;

FIG. 2 is a similar view through another embodiment having a valve for releasing the control fluid from a control chamber which causes the first piston to move in its open position by opening the control fluid feed;

FIG. 3 is a similar view of another drum wherein the control chamber controlling the first piston is drained of control fluid by means of a bore rather than a valve when the feed of control liquid is shut off; 7

FIG. 4 is a similar view of another valve-operated embodiment of separator which gives an impulse to a control system outside of the separator for initiating desludgings in any desired manner; and

FIG. 5 is an enlargement of a modification of FIG. 4

showing how a magnetic or electric impulse can be established during operation.

Referring now more particularly to the drawings, in FIG. I there is shown a centrifugal separator whose drum comprises a lower member 1 and a cover 2 held together by a threaded ring 3. A first piston 4 is disposed for axial displacement in the lower portion I of the drum; its upper edge 5 abuts a seal 6 in the closed position, shutting the interior 7 of the drum from the ejection orifice or orifices 8 in the periphery. With the first piston or control valve 4 there is associated an auxiliary or second piston 9, so that the weight of the load in the drum acts only on the auxiliary piston 9.

Between the auxiliary piston 9 and the control or first piston 4 there is defined a second chamber 10, and between the control piston 4 and the lower part 1 of the drum there is defined a first chamber 11. Both chambers l0 and 11 are in communication with the control fluid feeding system. In the lower portion 1 of the drum there is also provided a valve 12 which is equipped with a radially displaceable valve plate 13. When there is a sustained feed of control fluid through passage 14 to chamber 15, the valve plate 13 will be held in the end position illustrated. To enable the pressure on the back of valve plate 13 to be relieved when the delivery of control fluid is shut off, chamber 15 communicates with the outside through a calibrated bore 16. Through this bore the liquid is then removed by centrifugal force from chamber 15 and passage 14.

After the delivery of control fluid is shut off, the pressure acting on the front of valve plate 13 when the valve closing chamber 1 1 is filled or partially filled pro duces an outward displacement of the valve plate, thereby allowing the piston displacing fluid to escape from the chamber 11 through passages 17, 18, 19 and 20.

The auxiliary piston 9 is guided sealingly against the control piston 4 on the outside and against the drum hub 21 on the inside. lts outer upper edge is provided with a plurality of sluices 22. The upstanding portions between these sluices serve to seat the auxiliary piston 9 against the drum cover 2.

A tube 23 is inserted for axial displacement in the drum hub 21, and is provided at its upper end with notches 24. The lower portion 25 of this tube 23 can be made of softer material, such as hard rubber, in order to achieve a better seal, if necessary, between the tube and a sliding valve 26 which cooperates with it. The sliding valve 26 rests slidably on the drum spindle 27 and is biased by a spring 28 which is mounted below the spindle cap 29 which can also slide along spindle 27. The sliding valve 26 is provided with a plenum chamber 30 for the control fluid which is delivered through tube 31. The plenum chamber communicates with the tube 23 through a bore 32 and with the chamber 10 through the notches 24 of said tube. Beneath the plenum chamber 30 an annular passage 33 is provided in the valve body 26 and one or more bores 34 radiate from said annular passage and communicate with an annular chamber 35 and the passage 14 when the valve body 26 is in the uppermost position. Beneath the sliding valve 26 there is an additional chamber 36 from which passages 37 lead to the valve closing chamber 11. The bottom of additional chamber 36 is defined by an inserted resilient ring 38. A skimming tube 39 extends into this chamber 36.

The auxiliary piston 9 is partially covered by a cap 40 which is sealed from the inner chamber of the drum by means of sealing rings'41 and 42. A plurality of helical springs 43 uniformly distributed about the periphery of the bottom of auxiliary piston 9 can be provided for the purpose of correlating the force acting on the bottom of the auxiliary piston to the specific weight of the solids and thus to the force of sludge acting on the upper side of the auxiliary piston.

The drum in accordance with the invention operates in the following manner. After the separator is started up, the sliding valve 26 is in the position represented. It is urged tightly by spring 28 against part 25 of tube 23. After the control fluid feed is opened, the control fluid passes out of line 31 into the plenum chamber 30 and runs from there through the bore 32, the tube 23 and the lateral apertures 24 of the latter into chamber 10. In this manner the auxiliary piston 9 is tightly urged against the drum cover 2 and the control piston 4 is urged downwardly. After chamber 10 is filled, the control fluid passes from the plenum chamber 30 into the annular groove 33 and runs through bores 34, annular passage 35 and passage 14 to valve 12, so that the valve plate 13 moves into the position illustrated in which it keeps the piston closing chamber 11 sealed from the exterior. A small portion of the control fluid continually bleeds off from valve 12 through the calibrated bore 16.

After the passage 14 leading to the valve has also filled up, the control fluid passes out of annular groove 33 into the chamber 36 and from there through passages 37 into the piston closing chamber 11, so that the control piston 4 moves to its closed position. This displaces inwardly a portion of the fluid from chamber 10, thereby increasing the overflow into chamber 36 and accelerating the closing of the drum. The feed of control fluid is sustained during the entire time of operation. Since the fluid pressure acting on the outside of the valve plate 13 of valve 12 is greater than the fluid pressure of piston closing chamber 11 acting on its inner side, the piston closing chamber remains closed off from the outside. The drum is then in the ready-tooperate state.

After the inlet is opened, the drum fills with the slurry to be separated, whose specific weight is not very different from that of the liquid comprising it; often the carrying liquid is water. Since the central portion of the upper side of auxiliary piston 9 is covered by the cap 40 affixed to the drum hub 21, the contents of the drum act only on the exposed portion of piston 9, so that the pressure acting on its lower side predominates and holds it in the position shown. As the deposit of solids progresses, the pressure on the upper side of the auxiliary piston increases. As soon as the required excess pressure is reached, the auxiliary piston moves downwardly. At the same time tube 23 and the sliding valve 26 are displaced downwardly by the inner part of the auxiliary piston, against the force of spring 28. This interrupts the communication between bore 34 and the annular passage 35, thereby also shutting off the flow of control fluid to valve 12.

After a portion of the control fluid has been thrown by centrifugal force out of passage 14 through the calibrated bore 16, the pressure acting on the inside of the valve plate 13 achieves predominance, so that valve plate 13 is displaced outwardly and the release of the piston closing fluid from piston closing chamber 11 through bores 17, 18, 19 and 20 begins. Since chamber 10, even when its volume increases, is always filled with fluid due to the uninterrupted feeding of the control fluid through line 31, the pressure in chamber 11' very rapidly falls below the pressure in chamber 10 when valve 12 is open. The control piston 4, therefore, soon moves downwardly and leaves an annular gap between its upper edge 5 and the sealing ring 6 for the ejection of solids.

As the depth of the solids diminishes, the force on the upper side of auxiliary piston 9 also diminishes, so that under the influence of the higher pressure in chamber 10 piston 9 moves upwardly again, while the sliding valve 26, under the force of spring 28, re-establishes communication between bore 34 and the annular passage 35, thereby first bringing valve 12 back into the closed position. The control fluid passing over into chamber 36 refills the piston closing chamber 11, and the control piston 4 moves back to the closed position.

As soon as the layer of solids in the sludge chamber of the drum again exercises the necessary pressure on the auxiliary piston, the above-described cycle is repeated.

Thus the drum in accordance with the invention automatically performs partial desludging operations when a certain amount of solids has collected in sludge chamber 7.

During the periods between desludging operations, the skimming member 39 extends into the fluid rotating with the drum, without carrying fluid out of chamber 36. This creates a static overpressure in the skimming member 39.

When the piston closing fluid is let out of chamber 1 1 during the desludging operation, the liquid level in chamber 36 drops, so that the overpressure in the skimming member 39 falls. This sudden drop of pressure can be transmitted as a pulse or signal to a control apparatus of known construction which, after a predetermined number of cycles, performs a complete desludging operation by interrupting the feed of control fluid through line 31 and performing all operations according to an established program.

In the embodiment shown in FIG. 2, a valve 12 is used for letting piston closing fluid out of piston closing chamber 11, this valve being normally closed, and being opened for the delivery of control fluid.

The valve body 50 is of such weight and its active surface areas are such that the closing pressure produced by the centrifugal force is greater than the fluid pressure in piston closing chamber 11 which tends to open it. The valve body 50 thus urges the valve plate 13 tightly against the outlet bores 20 during normal operation, so that the piston closing chamber 11 is sealed from the exterior. When control fluid is fed through passage 14 into the annular chamber 52, the valve body 50 is lifted from its seat and piston closing fluid is let out of the piston closing chamber 11 through passages 17, 18, 19 and 20. To relieve the fluid pressure in chamber 52, a discharge bore 51 is provided, through which the chamber 52 and passage 14 drain when delivery to passage 14 is shut off.

In this embodiment, the control fluid is. supplied through a line 48 which is constructed in the form of a skimming member, coming for example from an elevated tank. As in the embodiment shown in FIG. 1, the chamber is first filled with control fluid. The control fluid that continues to come in then passes from chamber 30 into chamber 30a and from there to chamber 36, and on through one or more passages 37 into the piston closing chamber 11. After this chamber is also filled, the free liquid level in chambers 30 and 36 rises toward the axis of rotation. Since the inside diameter of the disk 38 defining chamber 36 is smaller than the outside diameter of the skimming member 48, the skimming member 48 extends into the liquid. The pressure produced in this skimming member prevents inflow of control fluid.

When a certain amount of solids has accumulated in the sludge chamber 7 of the drum, the auxiliary piston 9 moves downwardly, whereupon the passage 37 leading to the piston closing chamber 11 is closed or greatly throttled and the chamber 30a is connected by bore 44 with the passage 14. The valve body 50 rises from its seat and control fluid is let out of piston closing chamber 11. At the same time the piston 4 moves into the open position and solids are is ejected from the drum through the apertures 8 in the drum periphery. As soon as the pressure acting on the upper side of the auxiliary valve 9 has become lower than the pressure acting on its lower side, auxiliary piston 9 moves upward. This again shuts off the passage 14 to the valve and the passage 37 to piston closing chamber 11 is reopened. When the fluid is centrifugally discharged from the annular chamber 52 and passage 14 through bore 51, valve 12 returns to the closed position, and the control fluid fed to piston closing chamber 11 also returns the main piston 4 to the closed position.

Upon the upward movement of auxiliary piston 9, the fluid level in chamber 30 drops back as a result of the increase in the volume of chamber 10 and the wider opening of passage 37, whereupon the pressure in the skimming member 48 briefly falls off. This pressure drop can be transmitted as a pulse or signal to a control member which, after receiving a preset number of pulses, performs a complete desludging by blocking the delivery of control fluid to skimming member 48 and governing delivery of control fluid to line 49, as well as performing other operations. By delivering control fluid from line 49 through chamber 46 and passages 47 and 14 to valve 12, the latter can be held open as long as desired and a complete emptying of the drum can be performed.

FIG. 3 shows an embodiment in which there is no valve for the discharge of piston closing fluid. The piston closing chamber 11 is provided with a calibrated drain bore 54 passing through the periphery of the drum, and fluid constantly is discharged through this bore. In order to hold piston 4 in the closed position during the centrifugal separating periods, an excess of piston closing fluid must constantly be delivered. For the purpose of automatic sludge ejection, the auxiliary piston 9 must momentarily interrupt or greatly throttle this delivery of fluid.

As in the embodiment shown in FIG. 1, the control fluid enters through a delivery line 31 which again first fills up the chamber 10. It then passes out of chamber 30 over into chamber 36 and passes from there through passage 37 into piston closing chamber 11. Chamber 36 is provided with a calibrated bore 55. The amount of control fluid delivered during operation must be slightly greater than the amount that emerges from the piston closing chamber 11 through bore 54. The correct setting is achieved when a little fluid emerges through bore 55. This assures that the closing chamber 11 will not become empty and that chamber 36 will not be filled with fluid.

When, after a certain amount of solids has collected in the sludge chamber 7 of the drum, the auxiliary piston 9 moves downward, and the delivery of control fluid to closing chamber 11 is interrupted or greatly throttled down, the piston closing chamber 11 starts to empty itself through the calibrated bore 54 and the piston 4 moves to the open position. A portion of the solids is then ejected through the openings 8 in the drum periphery.

As soon as the lowering of the level of solids reduces the pressure on the upper side of auxiliary piston 9 to less than the pressure acting on its under side, the auxiliary piston 9 moves back upward again. The control fluid again first fills chamber 10 and then piston closing chamber 11, so that the piston 4 returns to the closed position.

While the delivery of control fluid to piston closing chamber 11 is shut off or greatly throttled, the liquid level in chamber 36 has risen inwardly in chamber 36 to such an extent that the skimming member 39 dips into the liquid and transmits a pressure pulse to a control apparatus. After receiving a preset number of pulses, the control apparatus performs a complete desludging by blocking the flow of control fluid through line 31 and performing other operations according to a preset program.

The drum in accordance with the invention is completely automatic in operation as regards the performance of partial desludging. To this extent it needs neither a control apparatus nor the manipulation of any valve outside of the separator.

In FIG. 4 there is shown another embodiment which differs in only a few respects from that of FIG. I. In FIG. 4 there is no cap 40 partially shielding piston 9 and the construction of the sliding valve 26of FIG. 1 is somewhat modified.

A sliding valve 56 and the spindle cap 29 are slidably disposed on the drum spindle 27. By an inwardly projecting annular flange 57 there is formed in sliding valve 56 an upper chamber 58 which communicates through a bore 59 with the bore of tube 23 and through an additional bore 60 with an annular chamber 61. From the annular chamber 61 a passage 62 leads to the closing chamber 11 and passage 14 leads to valve 12.

The sliding valve 56, in conjunction with a resilient packing ring 63, forms another overflow chamber 64 into which skimming member 39 extends. The outer lower edge of sliding valve 56 is provided with openings 66. Through these openings and passages 67 and 68,

chamber 64 is emptied when, in the downward movement of sliding valve 56, the upper lip of the resilient packing ring 63 is pressed downwardly.

At the beginning of operation, control fluid is fed in through the feed line 31 and passes through an orifice into the chamber 58. From there it flows first through bore 59, and the lateral notches 24 in tube 23 into chamber on the underside of the auxiliary piston 9 and forces the latter upward. After chamber 10 is filled,

the control fluid passes from chamber 58 through bore- 60 into annular chamber 61. Through the inlet orifice of passage 14, which is located radially further out, first valve 12 is provided with control fluid and shifted to the closed position. A small portion of the liquid being fed to valve 12 constantly emerges from the valve through its calibrated bore so that it is held in the closed position by a constant inflow of control fluid. After passage 14 is filled up, the control fluid passes from annular chamber 61 through bore 62 into the closing chamber and forces the main piston 4 into the closed position shown in the drawing. After the closing chamber 11 has also been filled with fluid, the excess amount of fluid coming in passes over the annular flange 57 out of chamber 58 and into the overflow chamber 64, leaving the latter over the inside edge of the resilient packing ring. The skimming member 39 dips into the fluid rotating with the drum, without carrying fluid from the chamber 64. In this manner a static overpressure is produced in skimming member 39 by the rotating fluid.

As specifically heavier solids increasingly collect in the sludge chamber 7, the pressure on the inside of the auxiliary piston 9 increases. As soon as this pressure becomes greater than the pressure acting on its outside,

the auxiliary piston moves downward. Thus the tube 23, the sliding valve 56 and the spindle cap 27 are also forced downwardly against the bias of the springs 28. At the same time the sliding valve 56 also presses the upper lip of' the resilient packing ring downwardly, so that the chamber 64 is emptied through the openings 66 and the passages 67 and 68. Consequently, the pressure in the skimming member 39 falls, thereby imparting to the control apparatus the pulse needed to start the preset program. The program of the control apparatus also includes the shutting off of the control fluid being delivered through line 31. After valve 12 has emptied, the fluid pressure prevailing in the closing chamber 11 causes the valve to open and thus permits the escape of the closing fluid from chamber 11 through bore 17. At the same time the piston 4 moves to the open position and the drum ejects the solids accumulated in sludge chamber 7, letting them pass through the opened ejection apertures 8 at full rotary speed.

In FIG. 5 there is shown a coil 69 which is supplied with magnetic bias current through a line 70. Whenever the auxiliary piston 9 moves downward, the approach of part 71 of sliding valve 72 to the coil 69 produces a variation of the bias current. This variation can likewise be used as the starting pulse for a control apparatus.

As shown in phantom, the spindle cap 29 rotating with the drum can be provided with a ring 73 having a plurality of projections or recesses, which upon the downward movement of the auxiliary piston 9 comes into the plane of a coil 74 supplied with high-frequency electric current. In this manner the high-frequency voltage undergoes an amplitude modulation which after amplification can impart the starting pulse to the control apparatus.

It is also possible to use the fluid which is displaced inwardly from chamber 10 by the downward movement of the auxiliary piston 9 for the purpose of producing a pulse.

If desired, the auxiliary piston 9 can be given a shorter radial size so that it extends only partially into the sludge chamber.

It will be appreciated that the instant specification and examples are set forth by way of illustration and not limitation and that various modifications and changes may be made without departing from the spirit and scope of the present invention.

What is claimed is:

1. In a self-cleaning liquid-solids centrifugal separator having a rotatable drum with a sludge discharge orifice on the periphery thereof, and a displaceable first piston defining-with said drum a first chamber, and means for filling said first chamber with a fluid and for draining said first chamber of said fluid, filling and draining of said first chamber serving to displace said first piston so as to open and close said sludge discharge orifice during the operation of the separator: means for initiating a desludging operation comprising an auxiliary second piston extending into said drum and defining a second fluid control chamber with said first piston, means for admitting control fluid to said second chamber, said control fluid being adapted to support said second piston in predetermined position against the urging of material in said drum, the accumulation of a predetermined quantity of sludge in said bowl resulting in displacement of said second piston, said initi ating means further including means operatively associated with the displacement of said second piston to initiate a cycle of operations including draining of fluid from said first chamber, displacement of said first piston to open said sludge discharge orifice, discharge of sludge through said orifice, re-supply of fluid to said first chamber to displace said first piston and close said discharge orifice, and re-supply of fluid to said second chamber.

2. A separator according to claim 1, including a cap partially shielding said second piston from the material in said drum.

3. A separator according to claim 1, including a plurality ofsprings disposed between said first and second pistons and opposing the weight of material in said drum operating against said second piston.

4. A separator according to claim 1, including a valve controlling the presence of fluid in said first chamber, said valve being opened by displacement of said second piston so as to drain fluid from said first chamber and thereby permit displacement of said first piston and opening of said sludge discharge orifice.

5. A separator according to claim 1, including an open drain for fluid in said first chamber, fluid normally being supplied to said first chamber in sufficient amount to fill said chamber notwithstanding drainage so as to keep said first piston in position wherein said sludge discharge orifice is closed,-displacement of said second piston serving to reduce supply of fluid to said first chamber so that said first piston will be displaced to open said sludge discharge orifice.

6. A separator according to claim 1, including means for interrupting the supply of fluid to both said first and second chambers, whereby said first piston will be in position where said sludge discharge ofifice is open even if the force of material in said drum against said second piston falls below the level at which it would normally permit said first piston to return to the position in which said sludge discharge orifice is closed.

7. A separator according to claim 6, wherein said fluid supply interrupting means is actuated electrically by the cooperation of means operatively connected to said second piston, displacement of said second piston being sensed by said fluid supply interrupting means and actuating the latter either each cycle or after a predetermined number of displacements of said second piston.

8. A separator according to claim 6, wherein said fluid supply interrupting means includes a fluid reservoir holding the necessary amount of fluid in said apparatus to maintain said chambers in operative conditions, said reservoir including a drain which is normally closed but which is opened by displacement of said second piston. 

1. In A self-cleaning liquid-solids centrifugal separator having a rotatable drum with a sludge discharge orifice on the periphery thereof, and a displaceable first piston defining with said drum a first chamber, and means for filling said first chamber with a fluid and for draining said first chamber of said fluid, filling and draining of said first chamber serving to displace said first piston so as to open and close said sludge discharge orifice during the operation of the separator: means for initiating a desludging operation comprising an auxiliary second piston extending into said drum and defining a second fluid control chamber with said first piston, means for admitting control fluid to said second chamber, said control fluid being adapted to support said second piston in predetermined position against the urging of material in said drum, the accumulation of a predetermined quantity of sludge in said bowl resulting in displacement of said second piston, said initiating means further including means operatively associated with the displacement of said second piston to initiate a cycle of operations including draining of fluid from said first chamber, displacement of said first piston to open said sludge discharge orifice, discharge of sludge through said orifice, re-supply of fluid to said first chamber to displace said first piston and close said discharge orifice, and re-supply of fluid to said second chamber.
 2. A separator according to claim 1, including a cap partially shielding said second piston from the material in said drum.
 3. A separator according to claim 1, including a plurality of springs disposed between said first and second pistons and opposing the weight of material in said drum operating against said second piston.
 4. A separator according to claim 1, including a valve controlling the presence of fluid in said first chamber, said valve being opened by displacement of said second piston so as to drain fluid from said first chamber and thereby permit displacement of said first piston and opening of said sludge discharge orifice.
 5. A separator according to claim 1, including an open drain for fluid in said first chamber, fluid normally being supplied to said first chamber in sufficient amount to fill said chamber notwithstanding drainage so as to keep said first piston in position wherein said sludge discharge orifice is closed, displacement of said second piston serving to reduce supply of fluid to said first chamber so that said first piston will be displaced to open said sludge discharge orifice.
 6. A separator according to claim 1, including means for interrupting the supply of fluid to both said first and second chambers, whereby said first piston will be in position where said sludge discharge ofifice is open even if the force of material in said drum against said second piston falls below the level at which it would normally permit said first piston to return to the position in which said sludge discharge orifice is closed.
 7. A separator according to claim 6, wherein said fluid supply interrupting means is actuated electrically by the cooperation of means operatively connected to said second piston, displacement of said second piston being sensed by said fluid supply interrupting means and actuating the latter either each cycle or after a predetermined number of displacements of said second piston.
 8. A separator according to claim 6, wherein said fluid supply interrupting means includes a fluid reservoir holding the necessary amount of fluid in said apparatus to maintain said chambers in operative conditions, said reservoir including a drain which is normally closed but which is opened by displacement of said second piston. 